Japanese Patent Laid-open No. Sho 62-25290 discloses a conventional internal combustion engine carried on a motorcycle, a main shaft of a transmission is disposed rearwardly of a crankshaft, and a countershaft of the transmission is disposed rearwardly of the main shaft. Further, a pivot shaft on which a swing arm for supporting a rear wheel for rotation thereon is supported for rocking motion is disposed rearwardly of the countershaft. In other words, the shafts are all disposed at the substantially same height. The countershaft is an output power shaft, and the rear wheel is driven through a rear wheel driving sprocket wheel provided on the countershaft and a chain extending around the sprocket wheel. The countershaft is disposed substantially on a forward extension line of a line interconnecting the axle of the rear wheel and the pivot shaft of the swing arm so that no irrational interference may occur between the rocking motion of the swing arm and rocking motion of the chain.
Also a conventional example is available wherein a cartridge type transmission is formed using the rotary shaft arrangement described above. In this instance, the shift drum is provided below an intermediate location between the main shaft and the countershaft.
With the arrangement of the conventional transmission shafts described above, since the distance between the crankshaft and the countershaft is great, the position of the pivot shaft of the swing arm is retracted inevitably. If it is considered that the position of the crankshaft and the position of the rear wheel axle are fixed, then if the distance between the crankshaft and the countershaft is great, the distance between the pivot shaft of the swing arm and the rear wheel axle is small. In other words, it cannot be avoided to form the swing arm short. Where the swing arm is short, the rocking angle of the switching arm with respect to the same upward and downward oscillation is great, and a burden is imposed on a shock absorbing structure. Further, where the rocking angle is equal, if the swing arm is short, then the structure cannot sufficiently absorb great upward and downward oscillation of the rear wheel. Furthermore, if the swing arm is formed longer while the conventional transmission shaft arrangement is maintained, then the distance between the front and rear wheels increases.
From the reason described above, it is demanded to decrease the distance in the forward and backward direction between the crankshaft and the countershaft as much as possible to advance the pivotally supporting point of the swing arm on the vehicle body frame thereby to make the length of the swing arm great and sufficiently absorb upward and downward oscillation of the rear wheel to achieve stabilization of the vehicle body.